Using museum specimens of northern shortfin squid (Illex illecebrosus) to evaluate long-term ecological changes in the Northeast US Continental Shelf large marine ecosystem

Stable isotope analysis of museum specimens presents an opportunity to evaluate trophic relationships from historical surveys. After testing the effects of preservation methods on archived specimens, we analyzed beak tissue from 69 Northern shortfin squid (Illex illecebrosus) collected between 1873 and 1987 from the Northeast U.S. Continental Shelf Large Marine Ecosystem. Stable nitrogen (delta 15N) and carbon (delta 13C) isotopes were used to infer ecological changes in I. illecebrosus across historical periods spanning the pre-industrial era (late 1800s), subsequent periods of industrial fishing (1930-1960; 1960-1976), and widespread industry regulation (1985-1995). delta 15N values for samples collected in the 1900s were significantly depleted compared to values in the 1800s, with the highest delta 15N values occurring between 1985 and 1995. There was also a significant shift over time to more depleted delta 13C values. Illex illecebrosus occupied completely distinct isotopic spaces during the pre-industrial (1800s) and industrial (1930-1995) periods, suggesting a change in food-web linkages and habitat use over the last century. Changes in delta 15N indicated a shift to lower trophic-level foraging habits in the 1900's, while shifts in delta 13C may be associated with mechanisms such as the Suess effect and increased reliance on more pelagic resources. Isotopic niche breadth and diversity increased in I. illecebrosus over time, potentially indicating expanded resource use associated with anthropogenic impacts on regional trophic guild structure. Our results are the first to examine isotopic changes of I. illecebrosus over multi-decadal periods using museum collections, and among the first to track long-term changes in the trophic role of cephalopods in marine ecosystems.